The search for planets of the terrestrial type outside of the solar system, and the determination of their characteristics, is a central element of the astronomical scene. It is in response to a profound demand from the public.
Many missions adopt this as their objective, proposing the use of advanced techniques.
They use flying arrays of telescopes, whose light is combined in a central “hub”. Such an array can be used to cancel out the image of a star and to detect any planets that are close to the latter.
The desirable attenuation factor is of the order of a billion, and there is little doubt that these missions will not be launched before 2020.
A device that is simpler than a conventional space telescope is known from document FR 2 840 416. This document describes a device which, in particular, is lighter than a conventional telescope, and which includes a Fresnel lens associated with a remote detector.
This device is able to work only on light of the low coherence type. It is able to focus only very narrow-spectrum radiation whose focus is obtained by an appropriate distancing of the receiver, equipped with ad-hoc filtering where necessary.
The performance of this type of device is based on the fact that most of the light of a star is not focussed because it is not coherent, and because it is not on the right frequency. As an example, during an observation, it is possible to concentrate only on a transition of the ozone in order to detect an “interesting” planet.
Furthermore, the device provides very large apertures, of the order of 100 m in diameter, for example, since it is necessary only to deploy a membrane consisting of alternate transparent and opaque zones, whose flatness constraints are low.
Nevertheless, the device described in document FR 2 840 416 has drawbacks, and the prospects of deploying such a device remain remote.
In fact this is a new technology that is difficult to master. Construction of the Fresnel lens requires great precision, in particular regarding location of the opaque and transparent zones, and is relatively costly to create.